Retroreflective materials are characterized by the ability to redirect light incident on the material back toward the originating light source. This property has led to the widespread use of retroreflective sheeting for a variety of traffic and personal safety uses. Retroreflective sheeting is commonly employed in a variety of articles, for example, road signs, barricades, license plates, pavement markers and marking tape, as well as retroreflective tapes for vehicles and clothing.
Two known types of retroreflective sheeting are cube corner sheeting and microsphere-based sheeting. Microsphere-based sheeting, sometimes referred to as “beaded” sheeting, employs a multitude of microspheres typically at least partially embedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes or vapor coats, etc.) to retroreflect incident light. Cube corner retroreflective sheeting, sometimes referred to as “prismatic” sheeting, typically comprises a thin transparent layer having a substantially planar first surface and a second structured surface comprising a plurality of geometric structures, some or all of which include three reflective faces configured as a cube corner element.
Due to the symmetrical geometry of beaded retroreflectors, microsphere based sheeting exhibits the same total light return regardless of orientation, i.e., when rotated about an axis normal to the surface of the sheeting. Thus, such microsphere-based sheeting has a relatively low sensitivity to the orientation at which the sheeting is placed on a surface. In general, however, such sheeting has a lower retroreflective efficiency than cube corner sheeting.
Various types of prismatic retroreflective sheeting are known. For example, U.S. Pat. No. 5,200,851 to Coderre et al. describe prismatic retroreflective sheeting that retroreflects infra-red light but does not substantially retroreflect visible light. The cube-corner elements in the retroreflective sheeting include a polymeric matrix selected to be highly transmissive to infra-red light but substantially opaque to visible light.
U.S. Pat. No. 6,157,486 to Benson et al. describes a reflective film including alternating layers of at least a first and second polymer. The alternating polymeric layers are configured to exhibit a high reflectance for light within a first spectral range and a low reflectance for light within a second spectral range. One exemplary spectral range can be infra-red light and the other exemplary spectral range can be visible light.
U.S. Pat. No. 7,329,447 to Chirhart et al. describes a retroreflective layer having a first cap-Y value and a plurality of discrete pigmented indicia disposed thereon. The pigmented indicia define a second cap-Y value of the viewing surface of the sheeting, the second cap-Y value being less than the first cap-Y value.
PCT Publication No. WO 2007-005357 to Nakajima describes a transparent, wavelength-selective retroreflector that retroreflects light within a specific wavelength range and is transparent to visible light.
In recent years, the use of prismatic retroreflective sheeting has been investigated for a use in license plates. However, in at least some instances, use of prismatic retroreflective sheeting results in poor visibility and an inability to read the characters on the license plate due to halation (the spreading of light beyond its desired boundaries in a developed photographic image) when the retroreflective license plate is imaged in an automated license plate reader (“ALPR”) system.
ALPR systems detect and recognize a vehicle using an electronic system. Exemplary uses for ALPR include, for example, automatic tolling, traffic law enforcement, searching for vehicles associated with crimes, and facility access control. One advantage of ALPR systems is that they are can be used almost universally, since almost all areas of the world require that vehicles have license plates with visually identifiable information thereon. However, the task of recognizing visual tags can be complicated. For example, the read accuracy from an ALPR system is largely dependent on the quality of the captured image as assessed by the reader. Existing systems have difficulty distinguishing tags from complex backgrounds and handling variable lighting. One exemplary ALPR system is described in U.S. Pat. No. 7,387,393 to Reich et al. (counterpart to Japanese Patent Application Publication No. 2007-171956). ALPR systems typically use an infra-red camera and an infra-red light source that emits light rays that are incident upon the license plate. The infra-red camera and/or infra-red light source in many ALPR systems is located above or in the vicinity of the road. Consequently, the infra-red light emitted by the camera and/or light source is incident on the license plate at high entrance angles.